In an optical storage apparatus, such as an optical disc apparatus, a focus servo operation is carried out to focus laser light onto the recording film surface of the optical disc medium for accurate recording and reproduction. In the focusing servo operation, a focus error signal is fed back to control the position of the objective lens in such a way that the distance between the objective lens and the optical disc medium is kept constant.
In recent years, as the recording density increases, the diameter of the laser light decreases. Consequently, the distance between the optical disc medium and the objective lens tends to decrease.
The focus error signal functions as a linear error signal indicative of the position of the objective lens relative to the optical disc medium only when the distance between the optical disc medium and the objective lens is within an extremely narrow range of ±1 μm. To position the objective lens within this range where the feedback control is available, focus entry control has been conventionally employed. The focus entry control is carried out by detecting a signal level of the focus error signal, which is called the S shape, during an open-loop swing operation of the objective lens, sensing that the objective lens is within the linear range and switching to closed-loop control.
In the swing operation of the objective lens in the focus entry control, the objective lens is swung in such a way that the optical disc medium passes through the focal point of the objective lens without fail when the surface of the optical disc medium wobbles. If the focus entry operation fails in the swinging operation, the objective lens could come into contact with the optical disc medium. An example of a method for preventing this situation, a physical stopper is provided between the optical disc medium and the objective lens to physically limit the displacement of the objective lens. According to this method, even if the focus entry operation fails, the objective lens will not come into contact with the optical disc medium, so that data stored on the optical disc medium will not be destroyed.
However, when the focal length is significantly short, the gap between the optical disc medium and the objective lens in the focused state is equal to or smaller than several tens of micrometers. In this case, considering that the amount of surface deflection of the rotating optical disc medium is still several hundreds of micrometers, the stopper cannot be provided in the first place. To prevent collision between the objective lens and the optical disc medium, it is therefore essential to carry out the focus entry operation in a more reliable manner.
To reliably carry out the focus entry operation, the amount of overshoot after the focus servo control system is switched to a closed-loop mode needs to be within the coverage of the focus error signal. To this end, it is necessary to reduce the speed of the objective lens relative to the optical disc medium and the discrepancy between the actual and target positions immediately before the focus servo control system is switched to the closed-loop mode.
For instance, in the example described in Patent Document 1, a sensor that detects the position of the objective lens is provided on a focus actuator that moves the objective lens, and the distance between the objective lens and the optical disc medium is directly detected to achieve a stable focus entry operation. In this example, however, the position detection sensor needs to be mounted on the actuator, disadvantageously resulting in a larger, heavier actuator.
To address this problem, as a method for reducing the distance between the objective lens and the optical disc medium and the speed of the objective lens relative to the optical disc medium without providing the sensor for detecting the position of the objective lens on the actuator, there have been proposed various methods for utilizing and measuring the periodicity of the disc surface deflection, storing it in a surface deflection memory and using feedforward control to cancel the surface deflection (see Patent Documents 2-5).    Patent Document 1: Japanese Patent Laid-Open No. 11-120569    Patent Document 2: Japanese Patent Laid-Open No. 2000-339712    Patent Document 3: Japanese Patent Laid-Open No. 2001-319352    Patent Document 4: Japanese Patent Laid-Open No. 10-149614    Patent Document 5: Japanese Patent Laid-Open No. 2000-207750
Among the methods using a surface deflection memory, Patent Document 2 describes a method for canceling surface deflection by detecting the component that follows the surface deflection from a focus control output in the focus servo operation to determine the amount of surface deflection, storing the amount of surface deflection thus determined in the surface deflection memory, and performing appropriate feedforward control. In this method, however, to acquire the amount of surface deflection, it is necessary to successfully perform the focus entry operation and switch to the focus servo operation. Therefore, in the initial state where the amount of surface deflection has not been acquired, there is a risk of collision between the objective lens and the optical disc medium in the focus entry operation.
Patent Document 3 describes a method for determining the amplitude of surface deflection, storing it in the surface deflection memory and using the thus stored amplitude of surface deflection in feedforward control by progressively bringing the objective lens closer to the optical disc medium and detecting the range of objective lens positions where the focus error signal is detected. In this method, however, it is necessary to hold the objective lens within the range where the optical disc medium wobbles due to its surface deflection. There is therefore a risk of collision between the objective lens and the optical disc medium during the measurement of surface deflection. The same problem applies to other methods in which the amount of surface deflection is measured before the control is switched to the focus servo operation as described in Patent Documents 4 and 5.
The present invention has been made in view of the above circumstances and aims to provide an information reproducing apparatus capable of entering a stable focus servo operation, by measuring the amount of surface deflection of a storage medium, such as an optical disc medium, in the initial state where no focus servo operation has been performed while eliminating the risk of collision between the objective lens and the storage medium, recording the resultant information in a surface deflection memory and reproducing the information for feedforward control. The present invention also aims to provide a surface deflection measuring method for measuring the amount of surface deflection.